Invisible Air

Boundary Field of the Wind King. An invisible sword.
The sword's blade has been sealed off by wrapping it in multiple layers of wind that alter the refractive index of light.
Strictly speaking, it is magecraft, not a Noble Phantasm.
Because it prevents the enemy from determining its range, it is far more effective in close combat than its relative simplicity would suggest.
It increases the user's accuracy rate and defensive power, but should the enemy happen to have “resistance toward compensation in response to visual obstructions” then Invisible Air will just be a normal sword clad in wind.
Against opponents with the skill False Mind's Eye, or who are otherwise protected from visual illusions, it has no effect at all.
Saber treats it as her primary weapon, but as Kojirou correctly surmised it is nothing more than a sheathe meant to conceal her holy sword.

Fate/side material: Fate Encyclopedia